Clean up ansi_c_convert_typet's interface

[tautschnig]

The need to invoke read() was only specified in a comment. Make the
constructor take care of this. Also, make the constructor initialise all
members rather than leaving this to clear(), which is thus no longer
necessary. Finally, do not unnecessarily inherit from messaget (there is
no is-a relationship).

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.38% // Head: 78.33% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (93c14fe) compared to base (9100d23).
Patch coverage: 29.16% of modified lines in pull request are covered.

❗ Current head 93c14fe differs from pull request most recent head 794f6f7. Consider uploading reports for the commit 794f6f7 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5599      +/-   ##
===========================================
- Coverage    78.38%   78.33%   -0.05%     
===========================================
  Files         1647     1645       -2     
  Lines       190362   190341      -21     
===========================================
- Hits        149213   149112     -101     
- Misses       41149    41229      +80     

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_convert_type.cpp	80.04% <2.17%> (-0.38%)	⬇️
src/cpp/cpp_convert_type.cpp	72.09% <50.00%> (-0.64%)	⬇️
src/ansi-c/ansi_c_convert_type.h	100.00% <100.00%> (ø)
src/ansi-c/c_typecheck_type.cpp	77.34% <100.00%> (-0.03%)	⬇️
src/util/replace_symbol.cpp	77.31% <0.00%> (-4.47%)	⬇️
src/util/validate_types.cpp	90.00% <0.00%> (-3.34%)	⬇️
src/ansi-c/c_typecheck_code.cpp	77.36% <0.00%> (-2.81%)	⬇️
src/util/simplify_expr_int.cpp	86.03% <0.00%> (-2.05%)	⬇️
src/util/pointer_predicates.cpp	93.10% <0.00%> (-2.02%)	⬇️
.../contracts/dynamic-frames/dfcc_wrapper_program.cpp	98.36% <0.00%> (-1.42%)	⬇️
... and 68 more

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[peterschrammel]

Sounds like there should have been 3 commits, but ok.

[tautschnig]

Sounds like there should have been 3 commits, but ok.

I turned it into 2 commits, breaking up the smaller one of the two turned out to be rather cumbersome and it wasn't really clear anymore where to put which change.